1. Field of the Invention
The present invention relates to a system and a method for analyzing particles in compressed gases, a method for controlling the particle growth in the system and a method for determining the optimum temperature for individual gases under various conditions.
2. Description of Related Art
Continuous monitoring of particles in bulk distribution systems is common practice and extremely low particle levels are required. Similarly, there is a need to quantify and verify particle concentration in cylinder gases. However, the true particle content of compressed cylinder gases is more difficult to determine for several reasons. Full cylinder pressure is typically about 20 times greater than that of a pipeline. The increased pressure makes pressure reduction for particle sampling more difficult. Additionally, the pressure in a gas cylinder decreases with usage, which affects the detected particles. Thus, sampling techniques used for pipeline gases are not directly applicable to cylinder gases. Moreover, there are sampling artifacts associated with cylinder gas pressure reduction.
Particle detection and analysis in chemical gases is important in the microelectronics industry. The requirements of such particle counters were discussed by Wang and Udischas (Microcontamination 93 Conference Proceedings, pp. 465-472), which is incorporated herein by reference in its entirety. The three specific requirements for such particle counters are material compatibility, purgability, and operating pressure. One of the key requirements is to measure particles at full cylinder pressure which may be the saturated vapor pressure for gases packaged in the liquid phase, such as HCl, or at the supercritical state for gases packaged in the gas phase, such as CF.sub.4.
A pressure-balancing technique was developed by Wang and Udischas (U.S. Pat. No. 5,209,102, issued May 1993, which is incorporated herein by reference in its entirety) to prevent the negative effects of particle bursts associated with the opening of the cylinder valves. This particle sampling and analysis technique was implemented in Air Liquide filling centers and was used to optimize cylinder filling processes (Air Liquide Electronics Journal, December 1993, which is incorporated herein by reference in its entirety).
However, molecular clusters and nanometer particles undergo extremely frequent collisions with other molecules, clusters, or particles. The sticking probability after collision determines if and how the clusters and particles grow. The predictive equations for the sticking probability between nanometer particles and wire screens were given by Wang (Eqn. 6-8, Aerosol Sci. Tech., 18, 180-186, 1993, which is incorporated herein by reference in its entirety).
Field tests show high particle concentrations for many gases at a saturated vapor pressure or supercritical state. The particle counts remain roughly the same even when a high efficiency particle filter is placed upstream of the particle counter (See FIG. 1). This indicates that the registered particles are artifacts associated with particle formation from molecular clustering and/or condensation of vapor-phase species. Because of the high concentration of these artifact particles, the true particle contaminants in the chemical gases are masked and cannot be easily determined.
The art lacks a system and a method for analyzing these particle contaminants in gases, including gases at a saturated vapor pressure or supercritical state. The art also lacks a method for reducing the artifacts associated with particle formation from molecular clustering and/or condensation of vapor-phase species.